To obtain unlimited rare candies in the original Pokémon Red and Blue games, you need to use a Pokémon that can surf to swim up and down the edge of Cinnabar Island and encounter Missingno. For the later FireRed and LeafGreen releases, the only way to achieve this is to use a GameShark cheat cartridge.
- Play the game until you reach Cinnabar Island
From the start, simply play your copy of Pokémon Red or Blue as normal. You need to reach Cinnabar Island and be able to travel back to Viridian City before you can attempt the cheat.
- Place the rare candy as the sixth item in your inventory
Once you have reached the right point in the game, fly to Viridian City. Before you do anything else, make sure you place the rare candy as the sixth item of your inventory. If you fail to do this, the cheat will not work. You can move the rare candy by going into your item menu, pressing select on the rare candy in its original position and then pressing select on the sixth item to switch them.
- Speak to the bald man who trained you to use a Pokéball
As annoying as he was to speak to the first time, to get unlimited rare candies you will need to talk to the bald man in Viridian once again. He is located on the main path as you walk towards the forest. When he asks if you are in a hurry, select "No" on the menu and watch the tutorial on how to catch a Pokémon.
- Swim on the edge of Cinnabar Island
Use "Fly" to reach Cinnabar Island quickly, then, using a Pokémon who can surf, begin to swim up and down along the east side of the island. You need to stay on the red strip that separates the land from the sea.
- Encounter Missingno and run
Keep swimming until you encounter a Missingno. If you come across any other Pokémon, it is best not to waste time, so just run and continue the search. Upon entering battle with Missingno, you will see that he looks quite different from all the other Pokémon. When you find Missingno, simply run away. If you capture this Pokémon, your game may crash as clearly this isn't a legitimate Pokémon. As soon as you are out of the battle, you should have your unlimited rare candies waiting for you in slot six of the item menu.